Many different propeller constructions have been proposed in the past and are presently available. Some propellers which have been proposed incorporate a ring or shroud which surrounds the propeller blades and is fixed thereto so as to be rotatable with the blades. The aim of such shrouds is to direct energy rearwardly from the propeller, rather than losing energy as a result of centrifugal action. Such propellers have not proved particularly effective and often have substantially decreased efficiency compared to normal unshrouded propellers. In particular, excessive pressure can build up within the ring and furthermore, viscous drag which occurs about the ring as it rotates builds up a rotational boundary layer about the ring increasing the effective drag area of the propeller.
One such propeller is disclosed in International Patent Application No. PCT/AU91/00582. The propeller described in that document includes an annular ring which joins the tips of the blades of the propeller and leaves free a region only on the high pressure side of the blades. It has been found in practice that this propeller has some disadvantages.
Other propellers have been provided with tip or end plates at the end of the propeller blades for distributing the vortices from the blades so that minimum kinetic energy losses occur. Current tip and end plate designs have had limited success.